This is written by Tony Messina who lives in Las Vegas, in response to
the question, 'Why do bats hang up side down'

"My name is Tony. I'm a retired electronics engineer. I have spent part
of my retirement studying an animal that I have always found fascinating
and allusive ... the bat. I also spend time working with at-risk kids in
science - and bats are a subject that is always sure to be brought up.
(They usually call me the "bat man")
I've heard the "Upside Down" question many times, and offer you these
observations:
No one can really say for certain why bats came to hang upside down. It
is surely a trait that developed of a long period of time. One idea I've
read is that bats might have evolved the behavior when they became
flying mammals, since the upside-down posture makes it easy to launch
into flight by simply dropping and gliding.
But kids won't let you off that easy ... since birds also fly, yet perch
upright !!
Another thought is that it enables the bats to more compactly roost at
the tops of caves where they are out of the reach of predators ... but
some children are quick to point out that bats also hide in cracks in
rocks and tree bark - so they really don't NEED to be upside down !
So I add one more piece to the puzzle that seems to finally to satisfy
the question.
A bat has the ability to go into a form of hibernation at will ... this
is called TORPOR. When cold weather reduces feeding opportunities, and
threatens physical harm, the bat can opt to "sleep" through the cold.
When the bat is in torpor, it must use as little energy as possible, to
maximize the use of its body fat for surviving through the cold period.
It can't waste energy using muscles in its extremities to hold onto a
perch!
Over time, the bat has developed a special kind of foot joint that lets
it lock its foot in a tightly clasped position, with the weight of the
body keeping the foot locked. By hanging from a safe and secure perch,
and locking this joint, the bat can drift off into a torpor without
worry of falling. Since it does not need to use any muscle to hold it's
grip the bat can "sleep" for as long as its body fats can sustain it.
So, all of these puzzle pieces can help to explain why bats hang upside
down. This upside-down, locked-in posture enables bats to huddle
together in safety, preserving body heat, avoiding predators, and
hibernating until more favorable conditions allow feeding to resume. At
the same time, the posture does allow for the bat to drop free of the
crowded mass of the roost, and unfold its large wings and glide into
flight.
Over many years, bat species have developed a way of life that includes
a hanging posture as one of many survival skills. They have also
developed other special skills, like the ability to find flying insects
in the dark with ultrasonic echoes, or rotate their heads 180 degrees so
they can see behind them while they hang in their roosts. Their bodies
have adapted to allow them to fill a niche they have made for themselves
in the natural world. It also provides another marvelous aspect of
Nature's variation for us to wonder at.
I hope these comments will help you answer your children's questions. I
hope they always ask questions and wonder why...
Tony Messina"